Production of fibers containing mainly polypropylene

ABSTRACT

A process for producing fibers containing polypropylene as the main component by the method of melt spinning comprises melting a new binder-consolidated nonwoven and spinning the melt into new fibers.

This is a Division of application Ser. No. 08/241/583 filed on May 12,1994 , now U.S. Pat. No. 5,466,411.

The present invention relates to a process for producing fiberscontaining mainly polypropylene by the method of melt spinning.

The use of melt spinning for producing polypropylene fibers is generallyknown (cf. Ullmann's Encyclopedia of Industrial Chemistry, VCHVerlagsgesellschaft, 5th edition, 1987, Vol. A10, pages 615 to 623).

Polypropylene fibers are suitable inter alia for producing nonwovenswhich are obtainable by consolidating fiber webs. Alongside mechanicaland thermal web consolidation it is in particular consolidation of fiberwebs with binders which is of importance. Binder-consolidated fiber websare used in particular for automotive trim, especially in the form offloor coverings. The latter are also used elsewhere. As well as beingchemically consolidated such nonwovens are generally mechanicallyconsolidated, for example by needling.

It is known from Nonwovens Industry 14 (12), 1983, 12-24, that aqueousdispersions of polymers formed from predominantly olefinic monomers andminor amounts of α,β-monoethylenically unsaturated carboxylic acids aresuitable for use as binder for producing nonwovens based onpolypropylene fibers.

German Utility Models 9210376, 9210375 and 9210377 disclose processesfor recycling products based on polyolefins. However, these processesare unsatisfactory as regards recycling polypropylene fiber nonwovensbased on binders comprising aqueous dispersions of polymerspredominantly formed from olefinic monomers.

It is an object of the present invention to provide a more advantageousprocess for recycling polypropylene fiber nonwovens based on binderscomprising aqueous dispersions of polymers formed predominantly fromolefinic monomers.

We have found that this object is achieved by a process for producingfibers by melt spinning, which comprises melting a nonwoven consistingessentially of:

a) fibers containing at least 80% by weight of polymers containing atleast 95% by weight of propylene in polymerized form, and

b) from 5 to 60% by weight, based on the fibers a), of a polymericbinder containing at least 70% by weight of a C₂ -C₄ -olefin inpolymerized form, and spinning the melt into fibers.

Surprisingly, fibers produced in this way are very useful for making newnonwovens.

Preferably the fibers a) contain at least 90, particularly preferably atleast 95, % by weight of polymers containing at least 95% by weight,preferably 100% by weight, of propylene in polymerized form. Otherpossible constituents of the fibers a) include for example polymerscontaining copolymerized units of ethylene as the main monomer, inparticular polyethylene or copolymers of ethylene and propylene. Inaddition the fibers a) generally contain additives such as antioxidantsor photostabilizers. Frequently the fibers a) are also colored. In thiscase the fibers a) contain suitable colored pigments. Advantageously thepigments are incorporated into the starting melt of the fibers.

The amount used of the polymeric binder b), based on the fibers b), ispreferably from 10 to 40, particularly preferably from 10 to 20, % byweight.

The polymeric binder b) can be added to the fiber web for example in theform of a low melting powder or in the form of low melting fibers. Inthis case the consolidating effect is achieved by briefly heating themixture to above the melting temperature of the binder b) and thencooling down. However, the preferred way of applying the binder b) is inthe form of its aqueous dispersion. Appropriate aqueous polymerdispersions and their preparation are known, preference being given tothose aqueous polymer dispersions whose polymers do not require anyexternal dispersants to disperse them in the aqueous medium, ie.polymers which are self-emulsifying. These aqueous polymer dispersionsand their preparation are described for example in EP-A-24 034 andDE-A-3 420 168 and U.S. Pat. No. 4 613 679.

Particularly preferred aqueous polymer dispersions of binders b) areaqueous polymer dispersions of self-emulsifying polymers whose polymerscontain acidic groups, in particular carboxyl groups, in effectiveamounts for the purpose of self-emulsification. The self-emulsifyingpolymers particularly advantageously contain in polymerized form from 70to 90% by weight of at least one C₂ -C₄ -olefin (preferably a C₂ - or C₃-olefin) and from 10 to 30% by weight of at least oneα,β-mono-ethylenically unsaturated mono- and/or dicarboxylic acid of 3to 8 carbon atoms and/or the anhydride thereof.

Of these, those aqueous polymer dispersions are in turn preferred inwhich the carboxylic acid comonomers contain 3 to 5 carbon atoms.Particularly preferred carboxylic acid comonomers are acrylic acid andmaleic acid or the anhydride thereof. Of course, all or some of theacidic functions can be present in neutralized form. Suitableneutralizing agents are alkali metal hydroxides, amines, which may alsocarry hydroxyl groups, and in particular ammonia.

Very particularly useful self-emulsifying polymers b) contain from 70 to90% by weight of ethylene and from 10 to 30% by weight of acrylic acidin polymerized form.

Of these, in turn, are those polymers of advantage which are mown fromNonwovens Industry, 14 (12), 1983, page 16, FIG. 1, or from the BASF AGTechnical Information Bulletin Bodenbel age und Sch aume, Epotal ® DS4024 X, March 1993, Epotal DS 4024 X being a very particularlyadvantageous aqueous dispersion of polymer b). As a rule, the relativeweight average molecular weight of these polymers b) is from about15,000 to 25,000 and their melt viscosity, determined at 200° C. with anMC 10 from Physica in the following Couette arrangement:

inner cylinder: radius=7 mm; length=21 mm

outer cylinder: radius=7.6 mm; (→shear 0.6 mm)

at 10 revolutions per second, is within the range from 10 to 150 pa·s.

The application of the aqueous dispersion of the binder b), the solidscontent of which is usually from 20 to 30% by weight, to the fiber webcan be effected by saturation, spraying, foam application, nip padding,knife coating or printing. The film formation which the polymerdispersion undergoes in the course of drying brings about theconsolidation of the fiber web.

To recycle the bonded fiber web it is simply melted and spun in aconventional manner to form new fibers. Advantageously, the bonded fiberweb is first mechanically comminuted and densified by heating and thenmelted in an extruder, extruded and forced through spinnerets. Ofcourse, prior to spinning the melt can have the usual additives addedsuch as coloring pigments.

The fibers thus obtainable, whose staple length is generally from 40 to120 mm and whose fineness is normally from 10 to 70 dtex, are veryuseful for making new nonwovens. In the case of chemically consolidatednonwovens the binder to be used is essentially free of any restrictions.Preference is given to using a binder b).

The process for recycling polypropylene fiber nonwovens according to theinvention is advantageous over those recycling processes whose purposeis to dissolve the polymeric binder out of the nonwoven and to reuse theresulting fiber web in that there is no binder solution to dispose of.

If the new nonwoven is always made using binder b), the process of theinvention can of course be employed repeatedly in succession.

If the passage of a plurality of such cycles leads to a decrease in themelting point of the recycled fibers, these may be suitable for use asfusion binders for new polypropylene fibers.

Of course, the process of the invention also extends to those nonwovenswhich include as a further structural element a rearwardly appliedpolyolefin film or a rearwardly applied polyolefin in powder form, inboth cases in particular of polypropylene.

EXAMPLE

(the fiber fineness is reported as the linear density (mass per unitlength) in tex; 1 tex=1 g·10⁻³ m⁻¹)

a) Commercially available polypropylene staple fibers (staple length: 80mm) of differing fineness were measured by the method of DIN 53 834 todetermine the tenacity (cN/tex) and elongation at break (%, based on theoriginal length). Results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Fineness [dtex]                                                                            Tenacity (cN/tex)                                                                          Elongation (%)                                      ______________________________________                                        20.0         32.0         128.7                                               15.5         35.0         117.1                                               ______________________________________                                    

b) Needled fiber webs formed from the staple fibers a) in a weight of700 g/m² were impregnated in the nip of a roll padder with Epotal DS4024 X (25% strength by weight aqueous polymer dispersion of a copoylmerformed from 80% by weight of ethylene and 20% by weight of acrylic acid)and dried (drying temperature: 125° C.). The binder content of theresulting nonwovens was 30% by weight, based on the weight of thestarting fibers.

The resulting nonwovens were mechanically comminuted, densified byheating, melted in an extruder, extruded and forced through spinnerets.The dimensions were chosen in such a way as to produce staple fibershaving a fineness of 20.2 or 15.2 dtex and a staple length of 80 mm.

The mechanical properties of these recycled staple fibers weredetermined as in a). The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Fineness [dtex]                                                                            Tenacity (cN/tex)                                                                          Elongation (%)                                      ______________________________________                                        20.2         26.2         128                                                 15.2         32.8         127                                                 ______________________________________                                    

c) The recycled staple fibers of b) were used to produce nonwovens as inb). The quality of the resulting nonwovens was essentiallyindistinguishable from that of the nonwovens resulting in b).

Likewise essentially indistinguishable were the qualities of floorcoverings made from the nonwovens of b) and the nonwovens of c).

We claim:
 1. Fibers, which are produced by melt spinning, whichcomprises:a) melting a non-woven consisting essentially of:i) fiberscontaining an effective amount of polymers, said polymers containing atleast 95% by weight of polypropylene; and ii) from 5% to 60% by weight,based on the fibers i), of a polymeric binder containing from 70% to 90%by weight of at least one C₂₋ C₄ -olenfin, and from 10% to 30% by weightof at least one α,β-monoethylenically unsaturated mono- or dicarboxylicacid of 3 to 8 carbon atoms or both, or the anhydride thereof or amixture thereof; and b) spinning the melt into fibers.
 2. The fibers ofclaim 1, wherein said fibers i) contain 100% by weight of polypropylene.3. The fibers of claim 2, wherein said fibers i) contain 100% by weightof recycled polypropylene.
 4. The fibers of claim 1, wherein saidpolymeric binder ii) is used in an amount of from 10 to 40% by weightbased on the fibers i).
 5. The fibers of claim 1, wherein said polymericbinder ii) comprises from 10% to 30% by weight of at least oneα,β-monoethylenically unsaturated mono- or di-carboxylic acid of 3 to 5carbon atoms or both, or the anhydride thereof or a mixture thereof. 6.The fibers of claim 1, wherein said polymeric binder contains from 70 to90% by weight of ethylene, and from 10 to 30% by weight of acrylic acid,each in polymerized form.
 7. The fibers of claim 1, which have a stablelength of from 40 to 120 mm and a fineness of from 10 to 70 dtex.